Drying process for pulverulent material

ABSTRACT

A process for drying wet pulverulent material by utilizing, as the heat-producing medium, the vapor form of the wetting liquid itself at a temperature higher than the deterioration temperature of the material being dried. The system comprises a continuous process whereby the pulverulent material is fluidized and dispersed while being centrifugally propelled through an arcuate mill. The fluidization and dispersion of the particles provides maximum surface exposure thereof to the heat of the liquid vapor. This results in an almost instantaneous flash-drying effect which is endothermic in nature. This endothermic reaction almost instantly lowers the temperature below the deterioration point of the material being dried. In this manner, very high temperatures can be used for rapid drying even though such temperatures may be above the deterioration temperature of the material being dried, while the vapor form of the liquid contaminate, itself, is used both as the source of drying heat and as the motive force.

United States Patent Stephanoff 1 Oct. 24, 1972 [54] DRYING PROCESS FORPULVERULENT MATERIAL Primary Examiner-Frederick L. Matteson AssistantExaminer-Robert A. Dua [72] Inventor. llillcholas N. Stephanoff,l-laverford, Att0mey Arthur A. Jacobs [73] Assignee: Fluid EnergyProcessing 8: Equip- [57] ABSTRACT v v em companyflatfieldpa' A processfor drying wet pulverulent material by 2] Filed: June 18, 1970utilizing, as the. heat-producing medium, the vapor form of the wettingliquid itself at a temperature 21 A .N 47232 1 pp] higher than thedeterioration temperature of the Related US. Application Data materialbeing dried. The system comprises a continuous process whereby thepulverulent material is [63] ggg t z g My fluidized and dispersed whilebeing centrifugally an one propelled through an arcuate mill. Thefluidization and dispersion of the particles provides maximum sur- (g1.face exposure thereof to the heat of the li qui d vapon This results inan almost instantaneous fiashdrying [58] Flew of Search 57 57 feet whichis endothermic in nature. This endothermic reaction almost instantlylowers the temperature below the deterioration point of the materialbeing [56] References Clted dried. In this manner, very hightemperatures can be UNITED STATES PATENTS used for rapid drying eventhough such temperatures may be above the deterioration temperature ofthe 2,297,726 10/ 1942 Stephanofi ..34/10 material being dried, whilethe vapor form of the 3,190,867 6/1965 Oldwe1ler et al. ..34/10 liquidcontaminate, itself, is used both as the Source of 3,212,197 10/1965Crawford ..34/ 10 drying heat and as the motive force. 3,403,451 10/1968Stephanoff ..34/l0 3,418,305 12/ 1968 Payne et al. ..260/94.9 4 Claims,2 Drawing Figures za- I 26 '36 I II I l8 22, /0

PATENTED 0m 24 m2 lNVENW/P ATTORNEY DRYING PROCESS FOR PULVERULENTMATERIAL This is a continuation of co-pending application Ser. No.748,515, filed July 29, 1968, now abandoned.

This invention relates to a process for drying wet pulverulent material,and it particularly relates to a process of the aforesaid type whereintemperatures are utilized which are higher than the deteriorationtemperature of the material being dried.

It has, heretofore, been the accepted procedure to dry a material at atemperature that is lower than the deterioration temperature (meltingpoint, decomposition point, etc.) of the material for the obvious reasonthat such deterioration is to be avoided. As a result, the drying periodwas unduly extended beyond the time that would have been necessary ifhigher temperatures could have been used.

Furthermore, since the material required longer processing periods andtherefore had to remain in the system for longer periods of time, it wasnecessary to use larger and more bulky equipment to handle largerbatches than would have been necessary if the material could have beendried rapidly enough to pass quickly through the system. Since mostplants have space problems, where every bit of available space must beused, the bulk of the drying equipment was economically wasteful.

Another disadvantage of prior drying systems was the waste of the liquidwhich was removed during the drying process. Obviously, this liquid hadto be removed from the system in order to prevent re-wetting of thedried material.

It is an object of the present invention to overcome the aforesaidproblems and disadvantages by providing a drying process which iscapable of utilizing temperatures that are significantly higher than thedeterioration temperatures of the materials being dried, while avoidingany deterioration of the dried material.

Another object of the present invention is to provide a process of theaforesaid type wherein the removed liquid is itself utilized in theprocess both as a motive force and as a drying medium.

Another object of the present invention is to provide a system andapparatus for effectively and efficiently carrying out the aforesaidprocess.

Another object of the present invention is to provide a system andequipment of the aforesaid type which is less complex and much smallerin bulk than systems and equipment heretofore generally used for thispurpose.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following description when read in conjunction with theaccompanying drawings wherein:

FIG. 1 is a schematic view of a system for carrying out a processembodying the present invention.

FIG. 2 is a fragmentary schematic view of a modified form of part of thesystem of FIG. 1. The invention will hereinafter be described for thetreatment of a specific material, namely, pulverulent polypropylenewetted with liquid methanol. However, it is to be understood that thisis merely one exemplification of the invention and that otherpulverulent material wetted with other liquids may be processed in thesame manner within the scope of the invention.

In accordance with the present invention, a fluid energy mill isutilized wherein the raw material, such as, for example polypropyleneadmixed with liquid methanol, is fed into the mill and entrained inmethanol vapor passing into the mill in the form of tangentiallydirected jets. In the specific instance exemplified here, thepolypropylene has a softening temperature point of about 275 F, whereasthe methanol vapor, as it comes from the heater, is introduced into themill at a temperature of between about 300 to 310 F. This wouldordinarily cause softening and melting of the polypropylene; however,the jets of methanol vapor fluidize and disperse the polypropyleneparticles and, at the same time, cause them to circulate through themill which has an elongated oval shape.

The fiuidization and dispersion of the polypropylene particles providesmaximum surface exposure of these particles to the heat of themethanolvapor. As a result, there is a flash-drying of the polypropylenewhereby the liquid methanol is almost instantaneously vaporized. Sincethis vaporization is an endothermic reaction, and since the reactionis'almost instantaneous, the temperature in that part of the mill almostinstantaneously decreases to about 200 F, well below the softening pointof the polypropylene. There is, therefore, insufficient time for anysoftening of the polypropylene to take place. As the polypropylene andmethanol vapor continues to circulate through the mill, additional heatis dissipated so that the actual temperature of the methanol vaporexhausting from the mill is about F. j

The fluid energy mill utilized in this system is of the standard typewherein the tangential jets of gas or vapor carry the fluidizedparticles through an upstack under centrifugal force. The centrifugalforce separates the larger, heavier particles from the smaller, lighterparticles, the larger particles being on the outer periphery and thesmaller particles on the inner periphery of the circulating stream. Anoutlet duct is provided at the inner periphery of the mill, adjacent theupper portion thereof, and the smaller particles pass through thisoutlet duct while the larger particles are recycled for a further passthrough the mill. Since the larger particles possess a larger surfacearea, they are more apt to retain some residual liquid, and therecycling thereof is, therefore, advantageous since it provides furtherexposure to the flash-drying step.

The above-described fluid energy mill is provided in a closed circuit toeffect re-utilization of the methanol vapors and, therefore, increasedefficiency as well as saving of space.

Referring now to greater detail to the figures of the drawings whereinsimilar reference characters refer to similar parts, there is shown asystem comprising a fluid energy mill 10 having a plurality of inletnozzles 12 extending tangentially into the bottom portion of the millfrom a header 14. The header 14 is in fluid communication with a duct 16through which methanol vapor is passed into the header. A hopper 18having an inlet duct 20 is used to feed the raw materials (in thisinstance, polypropylene mixed with liquid methanol) into the mill. Ascrew conveyor 22 is shown within the duct 20, but any other conveyingmeans, including a gravity feed, may be substituted.

The mill 10 is provided, adjacent its upper end and on the innerperiphery thereof, with an outlet 24 from which extends an exhaust duct26. The duct 26 leads into the upper portion of a standard cloth-bagtype of dust collector 28. In the collector 28, the solid particles areseparated from the vapor, the particles passing down through a rotaryvalve 30, continuously operated by a motor (not shown), into a ductleading to a storage bin, while the vapors pass upwardly through a duct34.

The duct 34 leads into the inlet side of a blower 36 and is alsoconnected to a by-pass duct 38.

The blower 36 has a predetermined displacement to take only apredetermined amount of the vapors passing through the duct 34. Theexcess vapors pass through duct 38 to a standard type condenser 40having a cooling liquid, such as water, passing therethrough. A storagetank 42 is provided to receive the condensed methanol, and a reflux pump44 is connected in a reflux line 46 between the tank 42 and the upperend of the condenser tower. A pump 48 is provided to pump the liquidmethanol from the condenser to a storage area.

A blow-off valve 50 is provided in the duct 34 to vent the vapors whenthere is excessive pressure in the duct. Similar blow-off valves 52 and54 are provided for the condenser 40 and tank 42, respectively. Ashut-off valve 56 is provided in the conduit 38. This valve is shown asmanually operable but it may be operated by a solenoid or any otherdesired means.

The blower 36, with a predetermined displacement, as indicated above, isoperatively connected to a heater 58, which may be of any feasible typebut which is illustrated herein as a steam jacketed heater with an inlet60 for steam to flow into the jacket and an outlet 62 for removal ofcondensed steam. The inlet 60 is connected to a'source of steam (notshown) and the outlet 62 is connected to a pump or the like (not shown).

A duct 64 leads from the outlet end of the heater 58 and is connected atan elbow portion 66 with the duct 16. A shutoff valve 68, similar tovalve 56, is interposed in the duct 16. Also provided in the duct 16,adjacent the elbow portion 66, is a valved inlet 70 connected to asource of nitrogen or similar inert gas (not shown). A bleed duct 72having a shut-off valve 74 is also interposed in the duct 16.

In the operation of the above-described system, the process is startedby introducing nitrogen gas, heated to a temperature above thevaporization point of methanol and at a pressure slightly greater thanambient, through the inlet 70. At the same time, polypropylene powder,containing 40-50 percent by weight of liquid methanol and at atemperature of 70 F, is continuously fed into the mill by means of ascrew conveyor 22.

The fed material is entrained by the nitrogen gas and fluidized anddispersed in the manner described above, the lighter particles passinginto the collector 28 and the heavier particles being recycled throughthe mill. In the collector 28, the solids pass through the rotary valve30 and through the duct 32 to storage, while the vapors pass up throughthe duct 34. These vapors include the starting nitrogen gas pulse thevaporized methanol from the raw feed. The solids recovered in thismanner have less than 0.5 percent by weight methanol adhering thereto.

The blower 36, being pre-set to a selected displacement, pulls theamount of vapors corresponding to such displacement into the heater 58,the remaining vapors, now cooled and substantially free from solidparticles, passing through duct 38 into the condenser 40. At this timethe nitrogen flow is shut off, the nitrogen being used only for startingthe system, while the methanol vapors, under pressure of the blower 36,circulates through the heater 58, where it is heated to about 300-310 F,and through duct 64, back through duct 16 and into the header 14 to beused as the motive and heating fluid for the next cycle through the mill10.

The system operates under slight positive pressure, so that at no timeis there any danger of introducing air or any other contaminant into thesystem. In the particular process here described, about 3,300 pounds perhour of polypropylene, wet with 2,200 pounds of methanol, is treated toobtain a polypropylene product containing 0.3 to 0.4 percent by weightmethanol. In this process, approximately 6,000 CFM of methanol vaporcontinuously circulates through the drying system.

Although nitrogen gas is used to start the system, since the specificheat of methanol is considerably higher than nitrogen, the efficiency ofthe process utilizing methanol is far greater and, therefore, muchsmaller equipment is necessary than if nitrogen were used. The nitrogenhas been exemplified as used to start the system because, beingcompletely inert, it acts to initially flush out the system. It is alsoless expensive than methanol. However, once the system is in operation,the use of the methanol from the raw feed itself it obviously far moreeconomical than nitrogen would be.

It is, of course, possible to use methanol vapor, air or any othergaseous fluid not only to start the system but even to act to form therecycled heating fluid, if so desired and if the particular processcalls for it. For example, superheated steam or steam-air combinationvapors may be used for drying heat-sensitive food products such asgrains, starches, etc. Where water is the liquid contaminant, steam maybe advantageously used so that the vaporized water is itself utilized tovaporize the water in the raw feed. Another type of process requires theelimination of isopropyl alcohol from penicillin. Here, either air orthe alcohol vapor itself may be used as the vaporizing agent as well asthe motive force.

In general, by means of the present system, the contaminant liquid inany pulverulent raw feed can be removed by the heat generated by thevapor form of the contaminant liquid itself, even if this heat is higherthan the deterioration point of the pulverulent material being dried, aslong as the removal of the liquid is practically instantaneous(flash-drying) and is sufficiently endothermic to cause correspondinglowering of the temperature below the deterioration point of thematerial. This can, furthermore, be accomplished in continuous cycleutilizing substantially only the contaminant liquid itself as the vaporsource.

The blow-off valves 50, 52 and 54, are utilized in the ordinary mannerto vent the vapors of gases when the pressures in the lines are toogreat. The shut-off valve 56 is closed until sufficient pressure andflow builds up in the duct 34, after which it is opened to permit bypassflow through duct 38. The shut-off valve 68 is used, if desired, atstart-up or to stop the action of the system while the bleed valve 74 isused to bleed off the starting nitrogen or any excess fluid.

FIG. 2 shows a slightly modified form of the invention wherein therotary valve 100, similar to valve 30, is

connected to an outlet duct 102, similar to duct 32, which is in fluidconnection with a duct 104 leading to a blower 106. A duct 108 leadsfrom a source of heated nitrogen gas (not shown) into the blower 106.

By means of this mechanism, the nitrogen gas blown through the duct 14not only entrains the polyurethane particles passing from the collector28 and conveys them to storage, but also acts to dilute any methanolvapor that is entrapped in the pulverulent product.

Obviously, many modifications of the present invention are possible inthe light of the above teachings. It

is, therefore, to be understood that within the scope of the appendedclaims, the invention may be practiced, otherwise than is specificallydescribed.

The invention claimed is:

1. A method of rapidly removing liquid from wet pulverulent materialwhich comprises simultaneously fluidizing and c'entrifugally dispersingsaid material in an arcuately moving low velocity circulating stream ofgaseous fluid by inserting said material, at substantially ambientpressure, directly into said circulating stream and by flash drying saidmaterial during contact with said circulating stream of gaseous fluidwhile said fluid is at a temperature higher than the deteriorationtemperature of said material in an endothermic reaction during which thetemperature of said fluid drops to below said deterioration temperature,said fluid moving at a velocity which issufficient to convey anddisperse said material but which is insufficient to cause significantpulverization thereof, said material being separated into lighter andheavier particles by the centrifugal force generated by said arcuatelymoving circulating stream of gaseous fluid in such manner that theheavier particles are on the outer periphery and the lighter particlesare on the inner periphery of the arcuately moving circulating stream,centrifugally removing the lighter particles and entraining gaseousfluid from said circulating stream, separating the removed lighterparticles form the entraining gaseous fluid, reheating the separatedgaseous fluid to a temperature higher than the deterioration temperatureof said material and intermixing the reheated gaseous fluid withadditional pulverulent material while propelling said reheated fluid andadditional material into said arcuately moving circulating stream.

2. The method of claim 1 wherein said gaseous fluid is the vapor phaseof the liquid being removed.

3. The method of claim 1 wherein said lighter particles, after beingseparated from the heavier particles, are separated from entraininggaseous fluid by gravity differential.

4'. The method of claim 1 wherein the pulverulent material ispolypropylene and the gaseous fluid is methanol vapor.

1. A method of rapidly removing liquid from wet pulverulent materialwhich comprises simultaneously fluidizing and centrifugally dispersingsaid material in an arcuately moving low velocity circulating stream ofgaseous fluid by inserting said material, at substantially ambientpressure, directly into said circulating stream and by flash drying saidmaterial during contact with said circulating stream of gaseous fluidwhile said fluid is at a temperature higher than the deteriorationtemperature of said material in an endothermic reaction during which thetemperature of said fluid drops to below said deterioration temperature,said fluid moving at a velocity which is sufficient to convey anddisperse said material but which is insufficient to cause significantpulverization thereof, said material being separated into lighter andheavier particles by the centrifugal force generated by said arcuatelymoving circulating stream of gaseous fluid in such manner that theheavier particles are on the outer periphery and the lighter particlesare on the inner periphery of the arcuately moving circulating stream,centrifugally removing the lighter particles and entraining gaseousfluid from said circulating stream, separating the removed lighterparticles form the entraining gaseous fluid, reheating the separatedgaseous fluid to a temperature higher than the deterioration temperatureof said material and intermixing the reheated gaseous fluid withadditional pulverulent material while propelling said reheated fluid andadditional material into said arcuately moving circulating stream. 2.The method of claim 1 wherein said gaseous fluid is the vapor phase ofthe liquid being removed.
 3. The method of claim 1 wherein said lighterparticles, after being separated from the heavier particles, areseparated from entraining gaseous fluid by gravity differential.
 4. Themethod of claim 1 wherein the pulverulent material is polypropylene andthe gaseous fluid is methanol vapor.